Field of the Invention
The present invention relates to an image forming apparatus that forms an image by transferring and fixing a toner image onto a long paper sheet, and to an image forming system that includes the image forming apparatus.
Description of the Related Art
There have been electrophotographic image forming apparatuses such as printers and copying machines. Such an image forming apparatus includes a sheet feed path that guides a paper sheet in contact with a transfer belt to a transfer nip portion when a toner image is transferred onto the paper sheet. This is to prevent electric discharge at the time when toner is transferred onto the paper sheet.
A structure in which a paper sheet is brought into contact with a transfer belt has been suggested in the form of a technique of pressing a paper sheet against a transfer belt by winding the paper sheet around a transfer roller, with the pressed portion of the transfer roller serving as the transfer belt (see JP 2002-132965 A, for example).
In a structure that conveys a long paper sheet, tension is applied to the long paper sheet being conveyed, so that the long paper sheet will not sag. Therefore, it is difficult to send the long paper sheet into the transfer nip portion while keeping the long paper sheet in contact with the transfer belt. In view of this, the sheet feed path of long paper sheets is designed to extend along the transfer belt.
The tension of a long paper sheet varies depending on sheet class, such as the sheet type or the basis weight of the long paper sheet. As the tension of a long paper sheet varies, the amount of deformation of the transfer roller facing the transfer belt also varies. Even if the sheet feed path of long paper sheets is designed to extend along the transfer belt, electric discharge before the transfer nip might occur in the sheet feed path. Such electric discharge before the transfer nip might further lead to an image defect due to discharge ununiformity.